The principal aim is further elucidation of the immunogenetic and phylogenetic basis of allograft incompatibilities. This will include several distinct, but conceptually related projects as follows: (1) Immunologic phylogeny and phylogeny of transplantation reactivity. This major focus will involve continued studies at three levels: lower invertebrates (esp. scleractinian corals), advanced invertebrates (esp. echinoderms), and primitive fishes (esp. hagfish). (2) Weaker histocompatibility barriers in congenic and recombinant strains of mice. The influence of single, known gene differences on skin allograft survival, induction of serum alloantibodies, kinetics of alloimmune memory, and graft-vs.-host reactivity, will be evaluated. (3) Genetic control of the antibody response to simple haptens conjugated to autogenous protein carriers in congenic strains of mice. The identity and function of the H-2 complex genes regulating high vs. low antibody responses to the TNP hapten conjugated to mouse serum albumin will be determined. The influence of certain non-H-2 genes will also be tested.